1. Field of the Invention:
The present invention relates to a method of charging a suspension stabilizer for suspension polymerization, and more particularly, to a method of charging a suspension stabilizer for suspension polymerization in an aqueous medium of a vinyl monomer comprising vinyl chloride.
2. Description of the Related Art:
Use of a partially saponified polyvinyl alcohol as a suspension stabilizer in producing a vinyl chloride polymer by suspension polymerization in an aqueous medium has hitherto been known.
It has been found that, of partially saponified polyvinyl alcohols, one which has a low polymerization degree and a low saponification degree improves markedly the quality with respect to the monomer removability, plasticizer absorption property, processability and the like of the vinyl chloride polymer obtained, and it has ben proposed to use a partially saponified polyvinyl alcohol having a relatively low average polymerization degree and a relatively low saponification degree (see Japanese Unexamied Patent Publication (KOKAI) Nos. 5886/1977, 115890/1977, 43792/1978, 6392/1978, 136089/1978 and 112210/1980). For example, Japanese Unexamined Patent Publication (KOKAI) No. 43792/1978 describes the use of a partially saponified polyvinyl alcohol having a polymerization degree of 500 or less and a saponification degree of 60 mole % or less.
However, such a partially saponified polyvinyl alcohol having a low polymerization degree and a low saponification degree is slightly soluble in water, and, accordingly, the partially saponified polyvinyl alcohol is charged into the reaction system prior to polymerization either directly as a powder or in the form of solution in an organic solvent such as methanol and acetone.
In the method of charging the partially saponified polyvinyl alcohol as a powder, however, it is necessary for developing a quality-improving effect on the polymer sufficiently, to preliminarily mix thoroughly the powdery suspension stabilizer with water and the monomer prior to polymerization. This procedure, however, takes a long time, leading to longer production time and lower productivity. On the other hand, in the method of charging the partially saponified polyvinyl alcohol as a solution in an organic solvent, the solvent thus used will remain in the polymer product obtained, thereby adversely affecting the quality of the product, or will be contained in waste water discharged after the polymerization reaction, thereby a complicating step of treating the waste water being required.